Zyxel OLT2412 [110/326] Multiple stp

Zyxel OLT2412 [110/326] Multiple stp
Chapter 13 Spanning Tree Protocol
OLT2412 User’s Guide
110
On each bridge, the bridge communicates with the root through the root port. The root port is the
port on this OLT with the lowest path cost to the root (the root path cost). If there is no root port,
then this OLT has been accepted as the root bridge of the spanning tree network.
For each LAN segment, a designated bridge is selected. This bridge has the lowest cost to the root
among the bridges connected to the LAN.
13.1.2 How STP Works
After a bridge determines the lowest cost-spanning tree with STP, it enables the root port and the
ports that are the designated ports for connected LANs, and disables all other ports that participate
in STP. Network packets are therefore only forwarded between enabled ports, eliminating any
possible network loops.
STP-aware switches exchange Bridge Protocol Data Units (BPDUs) periodically. When the bridged
LAN topology changes, a new spanning tree is constructed.
Once a stable network topology has been established, all bridges listen for Hello BPDUs (Bridge
Protocol Data Units) transmitted from the root bridge. If a bridge does not get a Hello BPDU after a
predefined interval (Max Age), the bridge assumes that the link to the root bridge is down. This
bridge then initiates negotiations with other bridges to reconfigure the network to re-establish a
valid network topology.
13.1.3 STP Port States
STP assigns five port states to eliminate packet looping. A bridge port is not allowed to go directly
from blocking state to forwarding state so as to eliminate transient loops.
13.1.4 Multiple STP
Multiple Spanning Tree Protocol (IEEE 802.1s) is backwards compatible with STP/RSTP and
addresses the limitations of existing spanning tree protocols (STP and RSTP) in networks to include
the following features:
Path Cost 1Gbps 4 3 to 10 1 to 65535
Path Cost 10Gbps 2 1 to 5 1 to 65535
Table 52 STP Path Costs
LINK SPEED
RECOMMENDED
VALUE
RECOMMENDED
RANGE
ALLOWED RANGE
Table 53 STP Port States
PORT STATE DESCRIPTION
Disabled STP is disabled (default).
Blocking Only configuration and management BPDUs are received and processed.
Listening All BPDUs are received and processed.
Note: The listening state does not exist in RSTP.
Learning All BPDUs are received and processed. Information frames are submitted to the learning
process but not forwarded.
Forwarding All BPDUs are received and processed. All information frames are received and forwarded.

Содержание